Will this work!! lol... I have some questions the return to water fall is elevated will the water go up it, or will it just fill up and overflow the trash can.. Feel free to drop some editing suggestions.
My system is about the same . The different are that I have external pump. Bins not trash can. I have 6 bins. The first filter I have an emergency pipe and a valve to drain dirty water. In your picture I already draw extra stuff you can check it out. The emergency pipe can help release water,so that the water not overflow the filter when you for got to clean it. My other bins are stacking with vertical Japanese mat and they are have emergency pipe and vales . On top of your carbon you could put on a layer of man made cotton. Cotton is pictured below. You may find it at a local fibrous store for about $3 to $4 a yard. This will catch any dirt before it go to next level. I do not have any fores rock in my filter because it very difficult to clean in long run. Do it right and get a good trash can or Bin will last a long time. Have fun with it.
Note, The emergency pipe should place close to return pipe is best. (pictured far away on the left side). Make sure there is clear opening at location of the return pipe so that clean water return to the pond with a good flow.
What should i put instead of lave rock. And cleaning world not be a problem at all, the seprate stages are seperated by eggcrate and can remove the very simply.
I too had a spray bar but after 3 years I removed it. Too easily clogged, so I just put in a T and have the water go in 2 different directions into the filter.